This might be splitting hairs a little, but that statement is actually incorrect.
The GTX 970 is a cut down version of the same GPU that is in the GTX 980, One thing that was "cut" from the GTX 980 happens to be a set of memory crossbars. So the 980 uses all of its 4GB at the same high speed. But the 970 can only use 3.5GB of its 4GB at the same high speed, and then that last half a gigabyte can only be accessed at 1/8th the speed. I will be the first one to admit that it takes a whole lot of things to happen to even get to using this last half a gigabyte, but on the other hand, going forward, it will happen more and more often with newer games.
So its pretty clear that any slowdown/stuttering problem the 970 has due to the half gig of slow memory access is not an issue with the 980.